Sirolimus is a drug being developed to prevent rejection after kidney transplant. The study is designed in a way that the patients will be either randomized to take Sirolimus instead of Mycophenolate, or they are randomized to local standard of care. The purpose of this study is to evaluate the long-term safety and tolerance of Sirolimus vs. the local standard of care. Patients must be 21 years of age or younger, and experience at least one episode of acute rejection. The patient must have stable renal function at the time of enrollment. Sirolimus is found to be more effective than local standard of care, then a way to stop or slow down the loss of the transplanted kidney will prevent the need for and cost of return to dialysis, loss from work, loss from school and frequent rehospitalization that is often associated with dialysis in children.